1. Field of the Invention
This invention relates to ozone generators and methods. More particularly, it concerns power supplies applicable to ozone generators, especially those using tubular electrodes, and methods of generating ozone therewith.
2. Discussion of the Prior Art
Industrial ozone generators using tubular electrodes are usually composed of a set of elementary generators connected in parallel within the same enclosure, each generator comprising two conducting electrodes separated by a narrow gap, through which a gas is passed, and a dielectric material, usually glass. These electrodes are concentric. Inside an outer metal electrode, around which cooling water circulates, a glass tube is placed. The glass tube is closed at one end and metalized internally, such metalization constituting the second electrode. A small discharge gap, e.g., 1-2 mm, is provided between the glass tube and the metal tube through which is passed either pure oxygen or a gaseous mixture, such as atmospheric air containing oxygen.
Between the two electrodes an increasing alternating potential difference, which is above a specified voltage corresponding to the breakdown voltage of the gas, is applied across the terminals of the generator. There appears in the discharge gap a violet corona resulting in the partial conversion of oxygen into ozone. Ozone forms when oxygen molecules are accelerated and collide in an alternating electric field. This formation only occurs when there is a voltage gradient and the electric field has reached the necessary strength to ionize the gas.
Ozone production by such generators is an increasing function of the electrical power applied thereto and the control of the production at the required value is, therefore, effected by adjusting said power.
In the past, significant refinements have been made to ozone generators and their integral power supplies. These refinements have been directed at increasing the efficiency of the generator or the power supply.
It is known that an ozone generator can be powered by a supply consisting of a saturable core reactor and an air gap transformer. This method as disclosed in U.S. Pat. No. 4,587,591 operates at line frequency, 50 or 60 hertz.
It is also known that the efficiency of an ozone generator can be increased by increasing the power supply frequency above the commercial AC line frequency. However, there is a maximum practical level for the frequency. In the past, these medium (50 to 1,000 hz) and high (above 1,000 hz) frequency power supplies have been voltage fed inverters. Since the power to each of the tubes must be controlled and the start of the corona is voltage dependent, it was obvious to adjust the voltage of the electric field. These power supplies have either had natural or forced commutation. The latter method is taught in U.S. Pat. Nos. 4,680,694 and 4,713,220.
Industrial generators which are made up of multiple elementary generators inherently have a large amount of stored energy which could feed through to one elementary generator in the case of a short circuit (broken dielectric tube). It is good engineering practice to fuse each dielectric tube to protect against short circuits.
Since the voltage source inverter supplies a voltage waveform to the load, the current is uncontrolled and determined by the load. Under faulting conditions of the load itself (ozone generator-broken dielectric tube), the resulting electrical short of the load will cause the current as supplied by the power supply to rapidly increase to uncontrolled destructive levels. This will result in a failure to commutate (turn-off) an inverter thyristor which then results in an internal short circuit within the inverter itself. This short circuit appears across the very large capacitor bank on the dc link which further results in fault currents. Thus, under faulting conditions of the load, a short circuit can cause uncontrolled destructive levels of currents to flow.
In order to shut down a voltage source inverter power supply under faulting conditions, the firing pulses to the inverter thyristors must stop. The current left flowing in the devices is influenced by the fault currents described above. In practice various types of protective schemes have been devised to self protect the voltage source inverter power supply, but each has some disadvantage. The result is that these schemes are only marginally successful and involve power components which result in additional circuit losses that result in lower efficiencies.
The major disadvantage of the voltage source power supply is its unreliability in not being able to control currents. This type of power supply conventionally has been air-cooled which limits operating the thyristor to near its full current rating. This method of cooling also opens the equipment cabinet to the environment and increases maintenance costs. Such prior art power supplies make use of several electromagnetic elements that have considerable weight and occupy a great deal of space. They are also relatively expensive.
The present invention provides improvements in the structuring and operation of ozone generators to mitigate or eliminate the defects of prior known devices as discussed above.
It is known that ozone generators can be powered by a current source inverter, compensation reactor and a high voltage transformer (see U.S. Pat. No. 4,790,980). This present invention make it possible to eliminate the need for a separate compensation reactor.